Polymer stabilized four domain twisted nematic liquid crystal display

ABSTRACT

A liquid crystalline light modulating pixel includes first and second cell wall structures and nematic liquid crystal within a UV cured polymer network disposed therebetween. The first and second cell wall structures cooperate with the liquid crystal to form four liquid crystal domains within the pixel. The liquid crystal in each of the domains exhibits a twisted nematic liquid crystal structure and the orientation of the liquid crystal director of the liquid crystal adjacent one of the cell wall structures in at least two domains is different. The polymer network stabilizes the four domain structure at both high field and zero field conditions.

This invention was made in part with government support undercooperative agreement number DMR-89-20147 awarded by the NationalScience Foundation. The Government has certain rights in this invention.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of copending U.S. Ser. No.08/445,174 filed May 19, 1995.

BACKGROUND OF THE INVENTION

This invention relates to the field of light modulating devices,particularly to a four domain twisted nematic liquid crystal pixel forsuch devices, and more particularly to a polymer stabilized four domaintwisted nematic liquid crystal display.

In liquid crystal flat panel displays, alignment of the liquid crystalmolecules is a major processing step. To obtain the required alignment,different types of alignment layers are deposited on the inner surfacesof cell wall structures of the display. The two major types of alignmentare homogeneous alignment, which is widely used in liquid crystaldisplays, and homeotropic alignment. In both types of alignment it isvery important to have a pretilt in one direction for the properoperation of the display. However, controlling the tilt angle withcurrently available homeotropic methods is difficult and notreproducible. Even though tilted homeotropic alignment provides highcontrast, the difficulty and time consuming procedures of currentlyavailable methods prevents its use in many industrial applications.

It is well known that twisted nematic liquid crystal displays, which arethe most common flat panel displays, have a narrow and nonuniformviewing angle and low contrast when viewed at off-angles from the normalwhen used in a gray scale mode. The electrooptical characteristics oftraditional twisted nematic liquid crystal displays are stronglydependent on the viewing angle. This presents a serious inconveniencefor good gray-scale operation. These drawbacks occur because all theliquid crystal molecules at the surface align unidirectionally and tiltin one direction.

To overcome these shortcomings of conventional twisted nematic liquidcrystal displays, two major approaches have been proposed, retardationfilm compensated liquid crystal displays and multi-domain twistednematic liquid crystal displays. Retardation film compensation improvesthe on-off electrooptical performance but does not solve the gray scaleproblem.

Concerning the multi-domain approach, recent analytical simulationssuggest that the best viewing characteristics in gray scale activematrix liquid crystal displays are obtained in twisted nematic liquidcrystal displays with four domains. Many techniques have been proposedin an attempt to develop multi-domain twisted nematic structures.Research has concentrated mostly on two domain liquid crystal displays,which suffer from a contrast inversion problem.

Many two domain tilted homeotropic displays are based on the fringefield effect. This technique requires special electrode designs andproper positioning of the domains. Because of the need for complexelectrode design, optical transmission in the bright state is low inthese displays.

Four domain liquid crystal displays have been fabricated using thefringe field effect and homeotropic alignment layers. These four domainliquid crystal displays suffer from the major drawback of requiring ahole in the electrode, which reduces the active area of the device.While the need for four domain twisted nematic liquid crystal displaysis great, no satisfactory implementation techniques have been presented.

A liquid crystal display employing tilted homeotropic alignment based onthe electrically controlled birefringence effect has high contrast nearnormal incidence, fast response times and satisfactory multiplexibility.For proper operation of such a display without defects it is veryimportant to obtain a tilted structure of the molecules in the samedirection. However, the required tilt angle must be very small, lessthan about 3°, to maintain a very dark off state and hence highcontrast.

K. Hiroshima and H. Obil SID Digest, p. 287 (1984), disclose a vacuumoblique evaporation technique that deposits layers of SiO_(x) onto arotating substrate to obtain pretilt angles of 0°-35°. This techniquerotates the substrate and does not enable a well-controlled pretiltclose to homeotropic. This publication also discloses that it ispossible to obtain homogeneous alignment with a pretilt of a few degreesby a rubbing process.

One widely used method to obtain tilted homeotropic alignment is twoangle deposition of SiO_(x) followed by an alcohol or organosilanetreatment of the substrates. In this method the pretilt is verysensitive to the thickness of the shallow angle SiO_(x) depositionlayer. Also, pretilt angles are highly dependent on the angle of SiO_(x)deposition in this method. This limits the size of the substrate thatcan be used.

In another method a homeotropic alignment agent is deposited on aunidirectionally rubbed surface. However, it is well known that therubbing process as an alignment technique can cause problems, especiallyin active matrix liquid crystal display applications.

Amorphous multidomain twisted nematic displays have been fabricated bynonrubbing methods, but have more than the optimum number of fourdomains. Amorphous multidomain twisted nematic displays commonly usechiral additives, which cause a unidirectional twist sense of thenematic structure. Even though the amorphous multidomain twisted nematicdisplays have satisfactory electrooptical characteristics, reverse tiltdisclinations lower their quality. A polymer stabilized amorphousmultidomain twisted nematic display has been proposed to stabilize thesedisclinations at the expense of lower contrast.

SUMMARY OF THE INVENTION

The four domain liquid crystalline displays of the invention arefabricated by simple techniques.

The four domain tilted homeotropic liquid crystal displays of theinvention also have a wide viewing angle. These displays do not sufferfrom the major drawback of bright disclination lines or domainboundaries on a dark background, and do not require an additionalprocessing step in which a black matrix is necessary to subdue lightscattered by the disclinations.

In the absence of polymer stabilization, the four domain structure isunstable at zero field and a pretilt angle larger than or about equal to10° is required to eliminate the instability. The energy barrier for twotwist states is too small to prevent one twist state to have a reversetwist effect. This effect causes a sub-pixel to have the same twistsense as its neighboring sub-pixels and destroys the four domainstructure. The polymer stabilized device of the invention has the stablefour-domain structures at both high field and zero field situations.

To effect polymer stabilization of the four domain structure, a smallpercentage of UV curable diacrylate monomer is added to the liquidcrystal material. These monomers form polymer networks after beingexposed to UV radiation. The formed polymer networks twist at differentdirections for the neighboring sub-pixels so that the interactionbetween the polymer network and liquid crystal molecules effectivelyincreases the energy barrier between left and right twist states. Withthe right amount of polymer network distribution in the liquid crystal,a zero field stable four domain device is obtained. Polymerconcentration affects both contrast ratio and brightness.

There are numerous liquid crystal alignment configurations orcombinations thereof suitable for use in the polymer stabilized fourdomain liquid crystalline light modulating pixel of the invention. Areverse rubbing treatment provides the liquid crystal molecules adjacentthe substrate with homogeneous alignment. A vacuum oblique evaporationtreatment provides the liquid crystal molecules adjacent the substratewith either homogeneous or homeotropic alignment.

As used herein, homogeneous alignment refers to when substantially allof the liquid crystal molecules adjacent a substrate in thehomogeneously aligned region lie generally parallel to one another, andsubstantially parallel to the substrate. Likewise, homeotropic alignmentrefers to when the liquid crystal molecules adjacent the substrate aregenerally parallel to each other and substantially perpendicular to thesubstrate.

As used herein the terms homogeneous and homeotropic alignment not onlyinclude when the molecules lie exactly parallel or perpendicular to thesubstrate, but also when the molecules are tilted with respect to thesubstrate or with respect to the substrate normal, respectively, suchthat they have a so called pretilt angle. Depending on the manner ofaligning the liquid crystal, the molecules may have a very slightpretilt angle inherently produced by the alignment procedure, or themethod and materials may be selected to intentionally provide a desiredpretilt angle.

The polymer stabilized liquid crystalline light modulating pixel of theinvention includes first and second cell wall structures and nematicliquid crystal disposed within a polymer network therebetween. The firstand second cell wall structures cooperate with the liquid crystal toform four liquid crystal domains within the pixel. The liquid crystal ineach of the domains exhibits a twisted nematic liquid crystal structure.

The orientation of the liquid crystal director of the liquid crystaladjacent the cell wall structures may be configured to produce thedesired four domains, depending upon masking pattern variations in thetreatment. For example, the orientation of the liquid crystal directorof the liquid crystal adjacent one of the cell wall structures in atleast two domains may be different; the orientation of the liquidcrystal director of the liquid crystal adjacent each of the cell wallstructures in at least two domains may be different; or the orientationof the liquid crystal director of the liquid crystal in all four domainsadjacent one of the cell. wall structures may be different.

If liquid crystal having positive dielectric anisotropy is used, theliquid crystal adjacent the cell wall structures is tilted with respectthereto from 0.5°-30°. Alternatively, if liquid crystal having negativedielectric anisotropy is used, the liquid crystal adjacent the cell wallstructures is tilted with respect to a normal to said cell wallstructures from 0.1°-10°.

Preferably, the liquid crystal director in two domains has right handrotation and the liquid crystal director in the remaining domains hasleft hand rotation. The liquid crystal director of the liquid crystal intwo adjacent domains adjacent one of the cell wall structures isoriented 180° with respect to the liquid crystal director in theremaining two domains adjacent that cell wall structure.

At a middle plane of the pixel substantially midway between and parallelto the first and second cell wall structures the liquid crystal directorin each domain is oriented orthogonal to the liquid crystal director inthe two adjacent domains. To obtain the desired orientations, each ofthe cell wall structures includes a rubbed polyimide layer or layers ofvacuum oblique evaporated material.

A liquid crystalline light modulating device constructed in accordancewith the invention includes first and second cell wall structures withnematic liquid crystal disposed within a polymer network therebetweenand means for electrically addressing the liquid crystal. The device hasat least one liquid crystalline light modulating pixel that includesfour liquid crystal domains within the pixel formed by cooperation ofthe first and second cell wall structures with the liquid crystal. Theliquid crystal in each of the domains exhibits a twisted nematic liquidcrystal structure. The orientation of the liquid crystal director of theliquid crystal adjacent one of the cell wall structures in at least twodomains is different. Other variations of the liquid crystal directororientation are possible, depending upon masking pattern variations inthe treatment, and will result in a four domain pixel, as discussedabove.

The light modulating device also includes two polarizers between whichthe cell wall structures are located. The polarization orientationpassed by one polarizer is either parallel to (parallel polarizers) ororthogonal to (crossed polarizers) the polarization orientation passedby the other polarizer.

In its broad aspects, the method of producing a four domain liquidcrystalline pixel for a light modulating device in accordance with theinvention includes the steps of providing first and second cell wallstructures. At least a first region on a cell wall structure is treatedto provide the liquid crystal director of the liquid crystal adjacentthe first region of the cell wall structure with an orientation in afirst direction. At least a second region on a cell wall structure istreated to provide the liquid crystal director of the liquid crystaladjacent the second region of the cell wall structure with anorientation in a second direction different than the first orientationdirection. The cell wall structures are spaced apart and a nematicliquid crystal is provided within a polymer network between the firstand second cell wall structures.

The treatment of the first and second regions, when properly oriented onopposing cell wall structures, produces four domains in which the liquidcrystal has a twisted nematic liquid crystal structure and theorientation of the liquid crystal director of the liquid crystaladjacent one of the cell wall structures in at least two domains isdifferent. The treatment may be varied to produce the other orientationsof the liquid crystal director discussed above.

One preferred treatment of the first and second cell wall structuresincludes forming a layer of material on the cell wall structures. Thelayer is rubbed in one direction. A mask is formed on the layer ofmaterial to form an uncovered portion and a covered portioncorresponding to the first and second regions. The first and secondregions extend throughout the entire substrate and preferably the entirelight modulating portion of a light modulating device. The masked layeris then rubbed in a different direction, and the mask is removed toprovide first and second regions rubbed in different directions. Therubbing direction of the first region is preferably opposite to therubbing direction of the second region. The cell is assembled by spacingapart the cell wall structures and orienting them such that the rubbingdirections on the first cell wall structure are orthogonal to therubbing directions on the second cell wall structure. Nematic liquidcrystal and a small percentage of UV curable monomers are filled betweenthe cell wall structures, followed by UV exposure to form a polymernetwork surrounding the nematic liquid crystal.

Another treatment of the first and second cell wall structures includesforming a first layer of material on the cell wall structures by vacuumoblique evaporation. A mask is formed on the first layer of material toform an uncovered portion and a covered portion corresponding to thefirst and second regions. The cell wall structures are rotatedpreferably 180° and a second layer of material is formed on them byvacuum oblique evaporation. The mask is then removed.

The cell is then fabricated by spacing apart the cell wall structuresand orienting them such that the evaporation directions of material onthe first cell wall structure are orthogonal to the evaporationdirections of material on the second cell wall structure. Nematic liquidcrystal and UV curable monomer are then filled between the cell wallstructures, followed by UV exposure.

In another embodiment, the fabrication method for producing a tiltedhomeotropic four domain display includes the steps of depositing a firstlayer of silicon monoxide onto a rotating substrate by vacuum obliqueevaporation. The rotation of the substrate is stopped opposite to thedesired pretilt direction and a second layer of material is deposited onthe substrate by vacuum oblique evaporation for a short period of time.

After the second evaporation the substrate is masked in an alternatingstriped pattern comprising first and second regions. Each stripe of themask is equal to about one half of the pixel size, i.e., the width ofone of the first and second regions. The substrate is rotated by 180°and a third layer of evaporated material is deposited on the fixedsubstrate for a short period of time. This provides liquid crystalmolecules adjacent the substrate surface with tilted homeotropicalignment with the tilt being in opposite directions in the first andsecond regions.

Prior methods for obtaining tilted homeotropic alignment require morethan one step and are lengthy procedures. In all these methods it is notpossible to control the pretilt angle below 4° very well which isimportant in obtaining displays having very high contrast. The primaryconventional method requires a final step of alcohol or organosilanetreatment of the substrate. Compared to these methods, the method of theinvention is much simpler and requires only the inorganic siliconmonoxide, which is very stable under many conditions and provides bettercontrol over the pretilt angle.

An advantage is that the deposition angle can be varied in the range of20°-70° with no difference in the final result. Thus, this methodprovides greater flexibility in preparing larger substrates compared toother SiO_(x) evaporation methods.

Another advantage of the tilted homeotropic display technique is that itdoes not require special electrode designs containing holes in the pixelelectrode, as do some current multi-domain displays. These currentdesigns require several high resolution photolithography steps. With thetechnique of the present invention it is much simpler to construct fourdomain displays and because of lack of an electrode hole, they have agreater active area.

The four domain tilted homeotropic displays of the invention have muchbetter electrooptical characteristics than conventional displays.Compared to conventional twisted nematic liquid crystal displays, tiltedhomeotropic four domain displays have much more symmetric viewing angledistribution, good gray scale capability and very high contrast.

In homogeneously aligned four domain displays, disclination lines due tothe reverse twist attributable to the low pretilt angle are a problem.Hence, the domain boundaries are not always stable. In the four domaintilted homeotropic display of the invention, disclinations or defectlines due to reverse twist are not present and domain boundaries arewell defined and very stable at any voltage applied across the display.

A further advantage of the present method for fabricating a tiltedhomeotropic four domain cell where rubbing is not required is thatsurface contamination and build-up of electrostatic charges are avoided.Thus, the tilted homeotropic displays of the invention are especiallysuitable for use as active matrix liquid crystal displays.

Further advantages of polymer stabilization of the four domainstructures of the invention include the features of low operatingvoltage and wide-viewing angle at both on and off states. Only oneadditional process step, the exposure of the filled cell to UVradiation, is added to the preparation of the cell. Polymer stabilizedfour-domain structures present a great potential for wide viewing angleActive Matrix Liquid Crystal Displays (AMLCDs). Because of low operatingvoltage, the production yield and life of AMLCDs can also be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a four domain homogeneous pixelconstructed in accordance with the invention, with the cylindricalmembers representing liquid crystal molecules and the arrowsrepresenting their pretilt direction.

FIGS. 2A and 2C are diagrammatic views showing two typical ways toassemble four domain pixels constructed in accordance with theinvention, with the dashed and solid arrows representing the rubbingdirections on the bottom and top substrates, respectively.

FIGS. 2B and 2D are diagrammatic views showing the liquid crystaldirector configurations at a middle plane m of the pixels of FIGS. 2Aand 2C, respectively, with the dashed arrows indicating the orientationdirections of the liquid crystal molecules in the middle plane m.

FIG. 3 is a representative drawing showing a conoscope image of a fourdomain homogeneous twisted nematic pixel constructed in accordance withthe invention.

FIG. 4 is a representative drawing showing the viewing anglecharacteristics of a four domain homogeneous twisted nematic cellconstructed in accordance with the invention.

FIG. 5 is a representative graph of a homogeneous four domain cell,showing the polar angle dependence of the transmission of 8 gray scales.

FIG. 6 is a representative graph showing transmission curves of fourdomain homogeneous cells constructed in accordance with the inventionhaving different resolutions, the sizes of each domain being, at 1 voltfrom highest to lowest transmission, a single domain, a 300 μm domainand a 50 μm domain, respectively.

FIGS. 7A,B are perspective views of a four domain tilted homeotropicliquid crystal pixel constructed in accordance with the invention atvoltage equal to 0 volts and voltage greater than V_(th), respectively.

FIG. 8 is a representative graph of a four domain tilted homeotropiccell, showing the polar angle dependence of the transmission of 8 grayscales.

FIG. 9 is a picture of a 2% polymer stabilized cell taken under amicroscope at the region of pixel size 100 μm×100 μm at 0 V.

FIG. 10 is a picture of a 1.5% polymer stabilized cell taken under amicroscope at the region of pixel size 100 μm×100 μm at 0 V.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings and to FIG. 1 in particular, a liquidcrystalline pixel for a light modulating device is shown generally at10. The pixel 10 comprises a liquid crystalline material 12 disposedbetween first and second cell wall structures or substrates 14, 16including a nematic liquid crystal. Only a portion of the first andsecond substrates 14, 16 corresponding to one pixel is shown in FIG. 1.Four domains d₁ -d₄ extend between the first and second cell wallstructures 14, 16 in which the liquid crystal 12 has a twisted nematicliquid crystal structure. FIG. 1 shows the twisted nematic structure ofthe liquid crystal molecules 18 of positive nematic liquid crystal inthe field-off state. The liquid crystal director of each of the liquidcrystal molecules 18 extends substantially along the long axis of themolecules 18. The liquid crystal director of the liquid crystal adjacentthe substrates 14, 16 preferably is tilted with respect to thesubstrates at an angle θ, which is commonly referred to as a pretiltangle.

The orientation of the liquid crystal director is determined by thetreatment technique, and is the direction in which the tilted directorpoints, as shown in FIG. 1 by arrows. The arrowheads depict the end ofthe director that is at a pretilt angle θ. The liquid crystal molecules18 adjacent the substrates 14, 16, either touch the substrates or arenear them. As used herein, reference to the surface liquid crystalmolecules as being oriented in a particular direction means a directionfrom the portion of the molecules nearest the substrate toward theportion of the liquid crystal molecules at the pretilt angle and furtherfrom the substrate.

When the liquid crystal has positive dielectric anisotropy, homogeneousliquid crystal alignment is employed in each domain, as shown in FIG. 1.The liquid crystal molecules 18 adjacent the cell wall structures 14, 16are tilted at angle θ with respect thereto from 0.5°-30°. When theliquid crystal has negative dielectric anisotropy, homeotropic liquidcrystal alignment is employed in each domain, as shown in FIG. 7. Theliquid crystal molecules adjacent the cell wall structures are tilted atan angle θ of 0.1°-10° with respect to the normal thereto.

The orientation of the liquid crystal director of the liquid crystalmolecules 18 adjacent the substrates 14, 16 may be configured to producea four domain structure in various ways. The orientation of the liquidcrystal director of the liquid crystal molecules 18 adjacent one of thecell wall structures 14, in at least two of the domains d₁ -d₄ will bedifferent, or the orientation of the liquid crystal director in all fourdomains d₁ -d₄ adjacent one of the cell wall structures 14, 16 will bedifferent.

However, in the preferred embodiment shown in FIG. 1, the orientation ofthe liquid crystal director of the liquid crystal molecules 18 adjacentthe cell wall structure 14 in two domains d₁ and d₃, is different thanin the remaining two domains d₂ and d₄. Similarly, the orientation ofthe liquid crystal director of the liquid crystal molecules 18 adjacentthe cell wall structure 16 in two domains d₁ and d₂, is different thanin the remaining two domains d₃ and d₄. The orientation of the liquidcrystal director in each of the domains d₁ -d₄ adjacent the cell wallstructures 14, 16 is homogeneous.

The liquid crystal molecules 18 adjacent the substrate 14 in the domaind₁ are in the same or a parallel plane as the liquid crystal molecules18 adjacent the substrate 14 in the domain d₂. However, the liquidcrystal molecules 18 adjacent the substrate 14 in the domain d₁ arerotated 180° about the cell wall normal, ie., opposite to, the liquidcrystal molecules 18 adjacent the substrate 14 in the domain d₂. Thissame relationship exists for the liquid crystal molecules 18 adjacentthe substrate 14 in the domains d₃ and d₄, and for the liquid crystalmolecules 18 adjacent the substrate 16.

Thus, the orientation of the liquid crystal director of the liquidcrystal molecules 18 adjacent each of the cell wall structures 14, 16 inat least two of the domains is different. The pretilt angle θ in allfour domains d₁ -d₄ is approximately the same. However, the inventioncontemplates having different pretilt angles in different domains.

The twist of the liquid crystal director 18 in the domains d₂, d₃ isright-handed (R) and the twist of the liquid crystal director 18 in thedomains d₁, d₄ is left-handed (L). It is known that chiral additivestend to produce a uniform twist sense in the twisted nematic structure.The liquid crystal of the invention does not have any chiral additiveand it is not necessary for the desired effect. An advantage of thedevice of the invention is that the invention utilizes an alignmentmethod that is simpler than that required to produce the same twist.

FIG. 2A shows the treatment of the pixel of FIG. 1, wherein a firstregion 20 on the cell wall structure 14 is treated to orient the liquidcrystal molecules 18 adjacent the wall structure 14 in one direction,and a second region 22 on the cell wall structure 14 is treated toorient the liquid crystal molecules 18 adjacent the wall structure 14 inanother direction. The lower cell wall structure 16 is similarly treatedto have the liquid crystal molecules 18 adjacent thereto oriented indirections 24, 26.

FIGS. 2A and 2C show two typical ways to assemble the cell. As seen inFIG. 2A, on each of the substrates 14, 16 the first regions 20, 24 arerubbed in a direction opposite to the direction of the second regions22, 26.

In FIG. 2C the substrate 14 has a first region 28 rubbed in onedirection and a second region 30 rubbed in the opposite direction,towards the first region 28. The other substrate 16 has a first region32 rubbed in one direction, and a second region 34 rubbed in theopposite direction.

As shown in FIGS. 1 and 2B,D, at a middle plane m of the pixelsubstantially midway between and parallel to the first and second cellwall structures 14,16, the liquid crystal molecules 18 in any givendomain (e.g., d₃) are oriented in a direction that is orthogonal to thedirection in which the liquid crystal molecules 18 in two domains (e.g.,d₄,d₁) is oriented.

In one embodiment of the invention a method of producing a liquidcrystalline pixel 10 for a light modulating device comprises the stepsof providing first and second cell wall structures 14, 16. As shown inFIGS. 1 and 2A, first regions 20, 24 on the cell wall structures 14, 16are treated to orient the liquid crystal director of the liquid crystalmolecules 18 adjacent the first regions 20, 24 in one direction. Thesecond regions 22, 26 on the cell wall structures 14, 16 are treated toorient the liquid crystal director of the liquid crystal molecules 18adjacent the second regions 22, 26 in an opposite direction than thefirst regions 20, 24. A nematic liquid crystal and a UV curable monomerare provided between the first and second cell wall structures 14, 16,followed by UV exposure.

The treatment of the first and second regions produces four domains d₁-d₄ in which the liquid crystal molecules 18 have a twisted nematicliquid crystal structure when the cell wall structures 14, 16 areproperly oriented. As seen in FIG. 1, the orientation of the liquidcrystal director of the liquid crystal 18 adjacent one of the cell wallstructures, e.g., cell wall structure 16, in at least two domains d₁, d₃is different.

In this embodiment, the treatment of the first and second cell wallstructures 14, 16 comprises a preferred reverse rubbing technique (FIG.2a) or a vacuum oblique evaporation technique. In the reverse rubbingtechnique the treatment includes forming a layer of polymer material,preferably polyimide, on the first substrate 14. The layer is rubbed ina first direction. A mask is formed on the first layer to cover one ofthe first and second regions 20, 22 and leave the other uncovered. Thefirst and second regions 20, 22 of the substrate 14 are now rubbed inthe opposite direction to the first rub. The mask is then removed,resulting in rectangular stripes comprising the first region 20 and thesecond region 22. The second substrate 16 is prepared in the samemanner.

In the vacuum oblique evaporation technique of this embodiment, thetreatment includes forming a first layer of SiO_(x) on the firstsubstrate 14 at an evaporation angle of preferably 85° from the normalto the substrate 14. A mask is formed on the first layer to cover one ofthe first and second regions 20, 22 and leave the other uncovered. Thesubstrate 14 is rotated 180° and a second vacuum oblique evaporation atpreferably 85° from the normal to the substrate 14 deposits SiO_(x) onthe first and second regions 20, 22 in the opposite direction. The maskis then removed, resulting in rectangular stripes corresponding to thefirst region 20 and the second region 22. The second substrate 16 isprepared in the same manner.

In both the reverse rubbing and the vacuum oblique evaporation methodsof the invention, the treatment of the first region 20 orients theliquid crystal director of the liquid crystal molecules 18 adjacent thesubstrate 14 in one pretilt direction. The treatment of the secondregion 22 orients the liquid crystal director of the liquid crystalmolecules adjacent the substrate 14 in a second pretilt directionopposite to the first direction. This provides the surface liquidcrystal molecules 18 in the region 20 with an opposite rotational sensethan the surface liquid crystal molecules 18 in the region 22. The firstcell wall structure 14 is spaced apart from the second cell wallstructure 16 and oriented such that the orientation direction of a givendomain (eg.d₂), on the first substrate 14 is orthogonal to theorientation direction on the second substrate 16 in that domain.

In another vacuum oblique evaporation embodiment of the invention, amethod for fabricating a tilted homeotropic four domain display includesthe steps of depositing a thin layer (300-500 Angstroms) of siliconmonoxide (SiO_(x)) onto a substrate 14 rotating at a given rpm (eg., 21rpm) by vacuum oblique evaporation. The surface topography of the layeris isotropic in the plane of the substrate due to the symmetry of thedeposition. The rotation of the substrate 14 is stopped opposite to thedesired pretilt direction and a second evaporation is given for a shortperiod of time. This adds a small anisotropy to the surface topographyand the liquid crystal molecules will tilt in that direction. Bychanging the thickness of the second layer it is possible to vary thepretilt angle from 0° to about 15°. It is also possible to obtainsimilar results by interchanging these two steps, i.e., firstevaporating on a fixed substrate, then evaporating on a rotatingsubstrate.

After the second evaporation the substrate 14 is masked in analternating striped pattern. The width of each stripe of the mask isequal to about one half of the pixel width. The striped mask leaves oneof the first and second regions 20, 22 covered and the other uncovered.The substrate 14 is rotated by 180° and maintained in a fixed position,and a third layer of evaporated material is deposited on it for a shortperiod of time. This provides liquid crystal molecules adjacent thesubstrate 14 with tilted homeotropic alignment with the tilt being inopposite directions in the first and second regions 20, 22. The secondcell wall structure 16 is treated in the same manner and spaced apartfrom the first cell wall structure 14. The first and second cell wallstructures 14, 16 are oriented such that the liquid crystal orientationdirection in a given domain on the cell wall structure 16 is orthogonalto the liquid crystal orientation direction on the first cell wallstructure 14 in that domain.

It will be appreciated by those skilled in the art that the inventioncontemplates conducting the reverse rubbing and vacuum obliqueevaporation treatments in different patterns. For example, instead ofusing alternating, parallel rectangular stripes in the photolithographyprocess, the substrates may be masked in a checkerboard fashion or thelike. To obtain the checkerboard orientation, the treatments are carriedout in a manner similar to that described, except the masking isperformed diagonally with respect to adjacent domains. Thus, in a fourdomain cell in the two regions that are diagonal to each other theliquid crystal is oriented in the same direction. This direction ispreferably opposite to that of the other two diagonal domains in thecell.

Combinations of the reverse rubbing process and the vacuum obliqueevaporation process are contemplated by the invention. For example, bothprocesses may be performed on the same substrate. Alternatively, cellsmay be constructed wherein the top substrate is treated by the vacuumoblique evaporation technique and the bottom substrate is treated by thereverse rubbing technique.

Suitable positive nematic liquid crystals include, for example, E7 andZLI 4792 manufactured by EM Industries. Suitable negative nematic liquidcrystals include, for example, ZLI 4330 and ZLI 2830, also availablefrom EM Industries. Other nematic liquid crystals and liquid crystalmixtures suitable for use in the invention would be known to those ofordinary skill in the art in view of the instant disclosure.

Suitable materials for the alignment layer used in the reverse rubbingtreatment would be known to those of ordinary skill in the art in viewof this disclosure and include, for example, polyimide. If the reverserubbing technique is used to fabricate the four domain twisted nematicliquid crystal displays, two key factors should be considered to selectthe suitable material, pretilt angle and compatibility to thephotolithography process. Of the many polyimide materials that have beentested, the best results are obtained using polyimide commonly availablefrom Nissan, which has the designation PI7311. This material has apretilt angle of about 8° using the liquid crystal E7. Alignment isstill good even with a subpixel size of 24×24 μm². A pretilt angle of 8°to about 9° is not large enough to stabilize the four domain structureat the zero voltage state. It is also desirable to select polyimide withlow baking temperatures, and then to bake it at high temperature. It hasbeen found that this reduces defects following the second rub.

In the absence of polymer stabilization, the four domain structure insome cases is unstable at 0 volts. For the liquid crystal E7, however,the structure is typically stable at voltages larger than 1 volt. Usingthe liquid crystal ZLI4792 instead of E7, the instability voltage isabout 1.6 volts. This may be because the liquid crystal pretilt angle islower if lower .increment.n liquid crystal material is used. One way tosolve this problem is to find a polyimide material that can produce alarger pretilt angle. To effectively increase the pretilt angle in atwisted nematic device, a polymer network may alternatively be used tostabilize the four domain structure using the technique described in ALow Threshold Voltage Polymer Network TN Device, SID Digest of TechnicalPapers XXIV, p. 887 (1993). For a more detailed discussion of the typeof polyimide to use for a particular liquid crystal pretilt angle andthe effect of rubbing on the orientation of liquid crystals adjacent thecell wall structures, see S. Kobayashi et al., New Development inAlignment Layers for Active Matrix TN-LCD's, IDRC Digest, p. 78 (1994),which is incorporated herein by reference in its entirety.

Suitable UV curable monomers include Desolite 2002-33 from DSM DesotechInc. and other such reactive materials known to those of skill in theart.

The means for electrically addressing the liquid crystal may includepatterned ITO electrodes on the glass substrates, active matrices suchas thin film transistors (TFT) and metal-insulator-metal type devices(MIM), and passive matrices. other addressing means suitable for use inthe invention would be known to those of ordinary skill in the art inview of the instant disclosure.

For a further discussion of the effect of polyimide on liquid crystaldirector alignment and the mechanisms of rubbing and SiO_(x) --obliquelyevaporated surfaces in liquid crystal devices, see LIQUID CRYSTALS,Applications and Uses, B. Bahadur Ed., World Scientific, Vol. 3, PP.45-55, 255-259 and 278-281 (1992), which is incorporated herein byreference in its entirety.

A better understanding of the features of the invention will be had fromthe following, nonlimiting examples.

EXAMPLE 1

When fabricating a four domain twisted nematic liquid crystal displaycell by the reverse rubbing process of the invention, an indium tinoxide (ITO) coated glass substrate was obtained from the Donnoly Corp.using their standard process. This clean, ITO coated substrate was spincoated at 3500 rpm for 30 seconds with a polyimide alignment material(Nissan PI-7311) to provide a polyimide layer of about 550 angstromsthick. The plate was then soft baked at 100° C. for one minute toevaporate the solvent, and then hard baked at 275° C. for two hours tocure the polyimide.

The substrate polymer that will cover the entire light modulatingportion of a light modulating display was rubbed uniformly in a firstdirection. This oriented the liquid crystal director of the liquidcrystal molecules adjacent the substrate in a first direction.

A photolithography process was then carried out to form a maskconsisting of parallel stripes on the substrate. The photoresistmaterial, Shipley S1400-31, was spin coated on the polyimide surface at3500 rpm for 25 seconds to deposit photoresist at a thickness of about1.8 μm. This was then baked at a hot plate for one minute at 100° C.

Then, an equal striped w mask (opaque and transparent to ultravioletlight) was used. The coated substrate was exposed to ultraviolet lightpassing through the UV mask at a power density of 3 mW/cm² for 1 minute,using the machine NuArc 26-1k, to cause one of the first and secondregions to be exposed and the other of the regions not to be exposed.The developer, Shipley MF312-CD27, was used to wash away alternatingstripes. The substrate was then washed with deionized water for 15seconds, blown with a nitrogen gun and baked on a hot plate at 100° C.for 1 minute.

After the photolithography process, the masked substrate was subjectedto the reverse rubbing process in which the covered and uncovered firstand second regions were rubbed uniformly in a second direction oppositeto, i.e., 180° from, the first buffing direction. This second rubbingoriented the liquid crystal director of the liquid crystal moleculesadjacent the substrate in the uncovered one of the first and secondregions in the second direction.

The photoresist mask was then removed with acetone and water to exposethe regions rubbed in the first direction. A second substrate wastreated in the same manner. The buffer conditions for the first andsecond rubbing were a plate moving speed of 3.5 ft/min, a drum rotationspeed of 550 rpm, a pile length of 1 mm. The process was repeated 2 to 4times.

A four domain pixel was obtained by spacing apart the substrates andcrossing them so that the rubbing directions of the first substrate wereorthogonal to the rubbing directions of the second substrate. The cellwas then assembled in the usual manner and filled with nematic liquidcrystal E7 which assumed a four domain twisted nematic structure betweenthe substrates. 1% UV curable monomer (Desolite 2002-33 from DSMDesotech Inc) was added and the cell was UV cured with 10 V applied toit. The cell had a thickness of about 6 μm, glass fiber spacers betweenthe substrates and patterned ITO electrodes. The cell .increment.ndsatisfied Gooch-Tarry's first minimum condition with an appropriatevoltage applied. Each subpixel or domain was approximately 1/32×1/32inch².

EXAMPLE 2

In fabricating a four domain tilted homeotropic cell using vacuumoblique evaporation, an indium tin oxide (ITO) coated glass substrate isobtained (Donnoly Corp.). Onto this clean, ITO coated substrate rotatingat 23 rpm a thin layer of SiO_(x) (150-600 angstroms) is deposited byvacuum oblique evaporation at an evaporation angle of 50°. An evaluationof the surface topography of the SiO_(x) surface by atomic forcemicroscopy will reveal a needle-like structure coming out perpendicularto the substrate. The surface features are isotropic in the plane of thesubstrate, which produce homeotropic alignment with zero tilt.

The rotation of the substrate is stopped at any direction and a secondevaporation for 3 seconds adds an SiO_(x) layer of about 20 angstromsthick to the substrate. This provides a small anisotropy to theneedle-like structure. On this surface liquid crystal molecules aretilted towards the opposite direction (in the plane of evaporation) ofthis second evaporation.

A photolithography process is then carried out to form a mask consistingof parallel stripes on the SiO_(x) layer deposited on the substrate. Thephotoresist material, Shipley S1400-31, is spin coated on the SiO_(x)surface at 3500 rpm for 25 seconds to deposit the photoresist at athickness of about 1.8 μm. The photoresist is then baked at a hot plateat 100° C. for one minute.

Then, an equal striped UV mask (opaque and transparent to ultravioletlight) is used. The coated substrate is exposed to ultraviolet lightpassing through the UV mask at a power density of 3 mW/cm² for 1 minute,using the machine NuArc 26-1k, to cause one of the first and secondregions to be exposed and the other of the regions not to be exposed.The stripe direction is in the evaporation plane, which contains thesubstrate normal and the evaporation direction. The developer, ShipleyMF312-CD27, is used to wash away alternating stripes. The substrate isthen washed with deionized water for 15 seconds, blown with a nitrogengun and baked on a hot plate at 100° C. for 1 minute.

After the photolithography process, the masked substrate is rotated by180° and maintained in a fixed position, and a third layer of evaporatedSiO_(x) is deposited on it for 4 seconds at a 50° deposition angle. Thisforms a 130 angstrom thick layer of SiO_(x) on the substrate. Thisprovides the liquid crystal director of the liquid crystal moleculesadjacent each substrate in the uncovered region with tilted homeotropicalignment. The tilt is in opposite directions, ie., away from eachother, in the first and second regions. A second substrate is preparedin the same manner.

The substrates are then spaced apart and filled with the liquid crystalmixture of 5% by weight ZLI 4330, 43% by weight ZLI 2806 and 1% monomer(Desolite 2002-33 from DSM Desotech Inc). The liquid crystal hasnegative dielectric anisotropy and is obtained from Merck. The cell isUV cured with 10 V applied to it. The cell has a thickness of 5 μm,glass fiber spacers, and patterned ITO electrodes. The subpixel width is24×24 μm². The cell And will satisfy Gooch-Tarry's first minimumcondition with an appropriate voltage applied.

EXAMPLE 3

A cell was constructed using a liquid crystal mixture of ZLI-1694(.increment.n=0.129) with 1.2% monomer (Desolite 2002-33 from DSMDesotech Inc.). The cell was UV cured with 10 V applied to it. For acomplete study, a series of 5.0 μm cells with four different polymerconcentration of 0.5%, 1%, 1.5% and 2% were then built. Liquid crystalmaterial ZLI-4792 (.increment.n=0.0969) was used this time to match thefirst interference minimum condition for a 5 μm thick TN cell. The cellsurface process consists of four steps. 1, rubbing two pieces ofpolyimide (Nissan PI-7311) coated glass substrates; 2, carrying outphotolithography (Shipley S1400-3) on the substrates to form a mask; 3,baking these two pieces of glass substrates for 2 hours at 250° C. and4, rubbing the substrates in the opposite direction and removing thephotoresist by acetone.

The two pieces of glass substrates were then put together with theirrubbing directions crossed. The sub-pixel size varied from 100 μm to 200μm. The cells were filled with liquid crystal material ZLI-4792 whichwas mixed with different monomer concentrations (Desolite 2002-33 fromDSM Desotech Inc.). The cells were exposed to UV light for 5 minutes (3mw/cm²) with an electric field of 10 V applied to it. FIGS. 9 and 10represent pictures of four domain structures for the 2% and 1.5% cellsat zero field.

The contrast ratio, drive voltage and response time data for all cellswere measured. As shown in Table 1, adding a small percentage of polymerdid not change the response time very much. The contrast ratio for the2% cell, even though is still very good, is a little bit smaller thanthe lower concentration cells. This effect is due to the bulkstabilization by the polymer network which causes the birefringence tobe just below the first interference minimum. A significant decrease ofdrive voltage with increasing polymer concentration is observed and thedata is shown in Table 2.

                  TABLE 1                                                         ______________________________________                                        Contrast ratio (CR) and response time for different polymer                   concentrations                                                                % of CR for  t (on) for                                                                             t (off) for                                                                          CR for       t (off) for                         poly-                                                                              o       o mode   o mode e     t (on for e                                                                          e mode                              mer  mode     ms!      ms!   mode  mode  ms!                                                                             ms!                                ______________________________________                                        0.5   83:1   15.0     7.0    100:1 13.0   7.0                                 1.0  143:1   13.0     5.0    100:1 13.0   6.0                                 1.5  143:1   15.0     5.0    125:1 17.0   5.0                                 2.0   67:1   25.0     5.0     50:1 30.0   6.0                                 ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Drive voltage for different polymer concentrations                                     V.sub.10 for O                                                                          V.sub.90 for o                                                                          V.sub.10 for e                                                                        V.sub.90 for e                           % of polymer                                                                           mode  V!  mode  V!  mode  V!                                                                              mode  V!                                 ______________________________________                                        0.5      1.9       3.0       1.9     3.1                                      1.0      1.9       2.9       1.9     2.9                                      1.5      1.3       2.7       1.5     2.7                                      2.0      0.8       2.6       0.8     2.6                                      ______________________________________                                    

This type of polymer stable four-domain TN LCD has the features of lowoperating voltage and wide-viewing angle at both on and off states. Onlyone additional process step, the exposure of the filled cell to UVradiation, is added to a reverse rub 4-D TN cell. The polymer stabilizedfour-domain TN presents a great potential for the wide viewing angleActive Matrix Liquid Crystal Displays (AMLCDs). Because of its lowoperating voltage, adopting PS4-DTN can also increase the productionyield and life of AMLCDs.

EXPERIMENTAL RESULTS

By analyzing the molecular configurations in each domain, it can be seenthat the liquid crystal device made by the reverse rubbing process ofthe invention has four domains. FIGS. 2A,C show two left hand and tworight hand domains. As shown in FIGS. 1 and 2A with respect to each ofthe two identical handedness domains, eg., domains d₂, d₃, the liquidcrystal molecules 18 adjacent the substrate 16 in the domain d₂ areoriented opposite to the liquid crystal molecules 18 adjacent thesubstrate 16 in the domain d₃. The liquid crystal molecularconfigurations at the middle plane m of each domain points to each offour quadrants. For example, as shown in FIGS. 1 and 2B, starting atdomain d₃ and moving to domain d₁, clockwise around the pixel 10, theliquid crystal director of the liquid crystal molecules 18 at themidplane m of each domain are 45°, 135°, 225° and 315°.

A four domain structure, obtained by the homogeneous rubbing technique,was confirmed by the conoscope image shown in FIG. 3 taken at 5 volts.However, suitable conoscope images could be obtained at 2-5 V. Theprinciples of microscopy that confirm the four domain structure areprovided in F. Donald Bloss, An Introduction to the Methods of OpticalCrystallography, p. 116, et seq., (1961). At high voltage, eg.,approximately 20 volts, each of the four domains had nearly the sameconoscope figure. At this voltage the liquid crystal in the cell washomeotropically aligned. Thus, the conoscope figures obtained at thathigh voltage (not shown) resembled those of materials having a centeredoptic axis, in which the location of the melatope was at the cross hairintersection and the bars of the uniaxial cross were bisected by thecross hairs. However, because of the twisted nematic structure in whichthe director is tilted in the center of the cell, the conoscope figuresat this voltage had some asymmetry.

When the voltage was removed or lowered to a level between the saturatedfull-off or full-on states of the device, the cross pattern at eachdomain moved away in a different direction that matched the directionsshown by the dashed arrows in FIGS. 2B and 2D. The center of the crossmoved into four distinct quadrants. This is because with the fieldlowered the cell returned to its pretilt alignment. When the optic axistilted into a particular domain, the center of the cross tilted intothat quadrant.

In the cells of the invention, although the distinctness of the crosseswas lost due to the twisted nematic structure, FIG. 3 clearly shows thatthe cross-like conoscopic figures have tipped into each of fourquadrants, ie., four different directions. This confirms that each ofthe domains was distinct and that a cell having a four domain pixelstructure was obtained.

Microscopy pictures of a test cell prepared by double SiO_(x) vacuumoblique evaporation under the normal white and black mode at zerovoltage showed disclination lines at the boundary of each domain. Whenvoltage was applied to the cell, the contrast of each domain alternatelychanged when viewing the cell at a fixed polar angle and varying theazimuthal angle. Because the molecular configuration was different inthe different domains, the domains mutually compensated each otheroptically and produced a wide viewing angle.

FIG. 4 shows the primary iso-contrast measurement under the normal whitemode with 6 volts applied in the on-state. The polar angle dependence oftransmission in eight gray levels of a four domain homogeneous test cellis shown in FIG. 5. The measurement results show no gray scale inversionwithin ±50° at any direction. These experimental results match well withcomputer simulations.

The effect of disclination lines at the boundary of the subpixels i.e.,domains, on contrast was investigated by measuring the transmissioncurves of four domain test cells with different resolutions. The resultsare shown in FIG. 6, which indicates that the disclination linesdecrease the transmission and the contrast, especially the transmission,in the full-off state as the resolution increases. This also shows thatat resolution higher than 100 μm light blocking stripes over thedisclinations are required to achieve contrast ratios greater than 220.

The required tilted homeotropic alignment (tilt angle ≈4°) was obtainedby the aforementioned method of producing four domain tilted homeotropiccells. A cell was fabricated with 5 μm spacers and two stripedsubstrates with the stripes oriented perpendicular to each other. Thisproduced a four-domain structure with each domain acting as anindividual tilted homeotropic twisted nematic cell as shown in FIG. 7A,B. The cell was filled with the liquid crystal mixture (43% by weightZLI2806+57% by weight ZLI 4330, .increment.n≈0.0938). Because of thevery small tilt angle, the domain boundaries were not visible in the OFFstate and the display was completely dark under crossed polarizers. Uponapplication of an electric field of sufficient strength the director inadjacent domains twisted in opposite directions as shown in FIG. 7B.This was confirmed by the directions of motion of the conoscopic crossin a sample with larger domains (4 mm²). The domain size was 24×24 μm²,i.e., the pixel size was 48×48 μm² such that the resolution was 500lines/inch. Reduction in contrast due to the dark domain boundaries wasvery minimal because the cells were dark in the OFF state.

The director configuration in each four domain pixel has two foldsymmetry about the center of the pixel. However, electro-opticcharacteristics have four fold symmetry. This is a great improvementover conventional liquid crystal displays.

The polar angle dependence of transmission in eight gray levels of afour domain tilted homeotropic test cell is shown in FIG. 8. Themeasurement results show no gray scale inversion within ±50° at anydirection.

It is important to note that to obtain wider viewing angles without grayscale reversal in both the homogeneous and tilted homeotropicembodiments, the optical path .increment.n.d is selected so the celloperates at Gooch-Tarry's first interference minimum condition, where.increment.n is the birefringence of the liquid crystal and d is thecell thickness. Decreasing the optical path below this first minimumvalue produces much wider viewing angles at the expense of brightness.

The aforementioned tilted homeotropic alignment technique of theinvention provides a much easier and simpler technique than existingmethods to obtain a four domain tilted homeotropic liquid crystallinepixel. Since the angle of SiO_(x) deposition is not very crucial in themethod of the invention, this provides the flexibility of makingalignment layers for larger displays. As an example, a 10"×10" substrateeasily fits into these limits at a distance of 14" from the SiO_(x)source.

Most importantly, changing the direction of the tilt angle withoutdamaging the alignment layer is quite easy in this method. Hence, thistechnique offers the possibility of making alignment layers usingphotolithography for other types of multi-domain displays. Also this isvery useful in making high contrast light valves for projectiondisplays.

The four domain tilted homeotropic display of the invention has theadvantages of wider viewing angle with high contrast, a very dark OFFstate and a bright ON state, good gray scale capability without anycontrast reversal (up to +60°) in both-horizontal and verticaldirections, no optical compensators and no rubbing treatment. This couldbe useful in direct view displays (conventional, passive and activematrix) which need a wide viewing angle, good gray scale capability andhigh contrast. In the four domain tilted homeotropic display of theinvention it is much easier than in conventional liquid crystal displaysto obtain very small domains (eg., 10 μm) without having jagged edges atthe domain boundaries. This is difficult to obtain by a rubbingalignment process. Because of the low switching times for all the graylevels this could be useful in displaying standard video rate imageswithout any difficulty.

The twisted nematic homogeneous alignment technique of the invention hasadvantages over the tilted homeotropic technique of the invention. Thereverse rubbing aspect of the twisted nematic homogeneous alignmenttechnique is easier from a manufacturing standpoint than the tiltedhomeotropic vacuum oblique evaporation technique. The twisted nematichomogeneous alignment technique also produces light modulating devicesthat are operable at a lower voltage and at faster switching speeds thandevices produced by the tilted homeotropic technique of the invention.

What is claimed is:
 1. A liquid crystalline light modulating pixelcomprising first and second cell wall structures and nematic liquidcrystal within a UV cured polymer network disposed herebetween, saidfirst and second cell wall structures cooperating with said liquidcrystal to-form four liquid crystal domains within said pixel, whereinthe liquid crystal in each of said domains exhibits a twisted nematicliquid crystal structure and the orientation of the liquid crystaldirector of the liquid crystal in at least two domains adjacent each ofsaid cell wall structures is different.
 2. The liquid crystalline pixelaccording to claim 1 wherein said liquid crystal has positive dielectricanisotropy and the liquid crystal adjacent said cell wall structures istilted with respect thereto from 0.5-30°.
 3. The liquid crystallinepixel according to claim 1 wherein said liquid crystal has negativedielectric anisotropy and the liquid crystal adjacent said cell wallstructures is tilted with respect to a normal to said cell wallstructures from 0.1°-10°.
 4. The liquid crystalline pixel according toclaim 1 wherein the orientation of the liquid crystal director in allfour domains adjacent one of said cell wall structures is different. 5.The liquid crystalline pixel according to claim 1 wherein the liquidcrystal director of the liquid crystal in two domains adjacent one ofthe cell wall structures is rotated 180° about the cell wall normal withrespect to the liquid crystal director in the remaining two domainsadjacent said one cell wall structure.
 6. The liquid crystalline pixelaccording to claim 1 wherein the four domain structure is stable at bothhigh field and zero field conditions.
 7. The liquid crystalline pixelaccording to claim 1 wherein the liquid crystal director in two domainshas right hand rotation and the liquid crystal director in the remainingdomains has left hand rotation.
 8. The liquid crystalline pixelaccording to claim 1 wherein at a middle plane of the pixelsubstantially midway between and parallel to said first and second cellwall structures the liquid crystal director in one domain is orthogonalto the liquid crystal director in two other domains.
 9. The liquidcrystalline pixel according to claim 1 wherein each of the cell wallstructures includes a rubbed polyimide layer.
 10. The liquid crystallinepixel according to claim 1 wherein each of the cell wall structuresincludes a layer of vacuum oblique evaporated material.
 11. A method ofproducing a four domain liquid crystalline pixel for a light modulatingdevice, comprising the steps ofproviding first and second cell wallstructures, treating at least a first region on said cell wallstructures to promote a liquid crystal orientation in one direction,treating at least a second region on said cell wall structures topromote a liquid crystal orientation in a different direction than insaid first region, spacing apart the cell wall structures and providinga nematic liquid crystal and a UV curable monomer therebetween, and,curing the cell by UV exposure while applying a field, wherein thetreatment of said first and second regions produces four domains inwhich the liquid crystal has a twisted nematic liquid crystal structureextending between said first and second cell wall structures and theorientation of the liquid crystal director of the liquid crystaladjacent one of said cell wall structures in at least two domains isdifferent.
 12. The method according to claim 11 further comprisingorienting the cell wall structures such that the orientation of theliquid crystal director of the liquid crystal in one domain adjacent oneof the cell wall structures is orthogonal to the orientation of theliquid crystal director of the liquid crystal in said one domainadjacent the other cell wall structure.
 13. The method according toclaim 11 wherein the treatment of said first and second regions of saidfirst and second cell wall structures comprises forming a layer ofmaterial on said cell wall structures, rubbing said layer in onedirection, forming a mask on said layer to cover one of said first andsecond regions and leave the other uncovered, rubbing said layer in saidfirst and second regions in a different direction, and removing saidmask.
 14. The method according to claim 11 wherein the rubbing directionof the first region is opposite to the rubbing direction of the secondregion.
 15. The method according to claim 11 wherein the treatment ofsaid first and second regions of said first and second cell wallstructures comprises forming a first layer of material on the cell wallstructures by vacuum oblique evaporation, forming a mask on said firstlayer to cover one of said first and second regions and leave the otheruncovered, rotating the cell wall structures, forming a second layer ofmaterial on the first and second regions by vacuum oblique evaporation,and removing said mask.
 16. The method according to claim 15 wherein thecell wall structures are rotated 180°.
 17. The method according to claim11 wherein said liquid crystal has positive dielectric anisotropy andthe liquid crystal adjacent said cell wall structures is tilted withrespect thereto from 0.5°-30°.
 18. The method according to claim 11wherein said liquid crystal has negative dielectric anisotropy and theliquid crystal adjacent said cell wall structures is tilted with respectto a normal to said cell wall structures from 0.1°-10°.
 19. The methodaccording to claim 11 wherein the orientation of the liquid crystaldirector in all four domains adjacent one of said cell wall structuresis different.
 20. The method according to claim 11 wherein the liquidcrystal director of the liquid in two domains adjacent one of the cellwall structures is rotated 120° about the cell wall normal with respectto the liquid crystal director in the remaining two domains adjacentsaid one cell wall structure.
 21. The method according to claim 11wherein the four domain structure is stable at both high field and zerofield conditions.
 22. The method according to claim 11 wherein theliquid crystal director in two domains has right hand rotation and theliquid crystal director in the remaining domains has left hand rotation.23. The method according to claim 11 wherein at a middle plane of thepixel substantially midway between and parallel to said first and secondcell wall structures the liquid crystal director in one domain isorthogonal to the liquid crystal director in two other domains.
 24. Themethod according to claim 11 wherein each of the cell wall structuresincludes a rubbed polyimide layer.
 25. The method according to claim 11wherein each of the cell wall structures includes a layer of vacuumoblique evaporated material.